hi my 420gsi 1993 has failed on the mot on exhaust emissions carbon monoxide content anyway when i looked at the sparkplugs they were black the fuel mixture or somthing needs looking at but i dont know where to start to fix this problem i need help please
Change the coolant temperature sender, (in the thermostat housing). They aren't the most reliable in the world. Are you also getting a high idle speed (above 1000rpm)? If so then it definatly sounds like a faulty sensor.
It's worth checking the vacuum pipe to the ecu is'nt split, this causes overfueling. The other common one is the cam timing a tooth or two out, they seem to run ok but it puts the manifold vacuum all to pot resulting in the same fault as above.